(a) Field of the Invention
This invention relates to a method of mounting semiconductor chips and more particularly, but not by way of limitation, to a method of mounting microwave, digital, analog chips and the like on a top portion of a metalized carrier substrate such as a mother board and making electrical connections thereto.
(b) Discussion of Prior Art
Heretofore, semiconductors have been used to manufacture microwave devices for many years. The use of newer materials, such as GaAs, have become increasingly important in recent years due to demands for better performance at higher frequencies. Currently, a large investment is being made into Monolithic Microwave Integrated Circuit (MMIC) technology which offers an efficient and cost effective method of integrating a number of complex functions onto a single chip. It is therefore becoming common to mount one or more complex circuits, e.g., a MMIC chip, rather than a single transistor, into either a small microwave hermetically sealed package or a hybrid assembly such as a Microwave Integrated Circuit (MIC). In either case, it is necessary to make electrical connections between conductors on the semiconductor and the package or carrier substrate. Except for beam lead devices, these connections are typically made with wire or ribbon bonds. These bonds generally introduce significant undesirable reactance which affects the circuit performance. Also, the reactance of these bonds varies from one circuit to another, especially when the bonds are made manually. On complex chips, a large number of bonds are frequently required, introducing performance uncertainty, high assembly costs and reliability concerns. The subject invention provides a method of mounting semiconductor chips eliminating the need for the above mentioned wire and ribbon bonds and the associated undesirable characteristics related thereto.
In U.S. Pat. No. 4,980,753 to Dunaway et al, U.S. Pat. No. 3,801,938 to Goshgarian, U.S. Pat. No. 3,768,050 to Stiles, Jr., U.S. Pat. No. 3,838,443 to Laighton, U.S. Pat. No. 4,415,867 to Rubin, U.S. Pat. No. 4,890,155 to Miyagawa et al., U.S. Pat. No. 4,901,041 to Pengelly, and U.S. Pat. No. 4,951,014 to Wohlert et al. various types of microwave circuits, packages and carriers are described. None of these prior art patents disclose the unique features, structure and method of mounting a semiconductor chip to a metalized substrate as described below.
In U.S. Pat. No. 4,626,805 to Jones, a surface mountable microwave IC package is described wherein transmission lines are printed on a printed circuit board. The transmission lines on the circuit board are connected to co-planar transmission lines on a backside of a carrier substrate. One of the transmission lines on the backside of the carrier substrate is connected to a topside microstrip line on top of the carrier substrate using a "through-hole" in the substrate. The "through-hole" being similar to a "via hole" as used in the description of the subject invention. To compensate for inductance added by the hole and the transmission line ends, a gap is adjusted to provide for compensation capacitance. The subject invention includes a microcover which attaches directly to the semiconductor chip providing for both hermetic sealing and shielding. While the subject patent describes similar structural features when compared to the invention described herein, such as the use of a through-hole or via hole, the similarities between the two inventions end here. Specifically, the Jones patent describes a microwave package and does not address the importance of providing a unique method as described herein of mounting and connecting semiconductor chips on a motherboard and the like through the use of properly indexed via holes on the substrate. Also, the Jones patent describes the use of a single through-hole in a carrier substrate received on top of a printed circuit board. The subject method of mounting semiconductor chips uses one or more via holes to connect a semiconductor chip on top of a metalized substrate such as a mother board. Further, the Jones patent does not address the importance and distinction of accurate via hole alignment and bonding for improved impedance transition from chip to substrate. Still further, the Jones patent discloses the use of wire bonds for interconnecting microwave chips which the subject invention eliminates.